1. Field of the Invention
The technical scope of the present invention is that of active mirrors, that is to say mirrors whose curvature can be modified to constitute a device to correct and focus a beam (for example a light beam, laser or other radiation).
2. Description of the Related Art
Active mirrors are essentially used in the optical domain (telescopes, microscopes) or in the domain of advanced research (synchrotrons). Active mirrors reflect an incident, oblique beam, normal to the mirror surface or inclined at any angle. In these specific domains, a mirror is sought to be deformed so as to focus the light beam, to correct an aberration of the beam or to correct a defect in the mirror.
The local modification of the slope of the mirror is moderate (around a few milliradians to one microradian). Moreover, the mirrors are generally made of a thin plate of a fragile material (for example silicon) whose thickness does not exceed 20 to 30 mm. It is, therefore, essential for the deformations of the mirror to be extremely accurate and precise (the stress applied being generally of between 1 and 100N).
Different active modification devices for the geometry of a mirror are known. A first type of device is that which implements piezoelectric or ferroelectric actuators. These actuators are spaced along and bonded to the mirror and enable a displacement of the portion of mirror to which they are bonded. Such devices nevertheless suffer from several drawbacks. A first drawback lies in the fact that piezoelectric actuators do not generate a stress but a displacement. Another drawback lies in the fact that keeping these actuators in position requires a constant supply of electricity and a closed loop control system. Another drawback is linked to the materials implemented, such as PZT ceramic, which generates a high level of hysteresis (of around 10 to 20%). Lastly, another drawback lies in the fact that the actuators are bonded on the mirror, thereby increasing the local rigidity of the mirror and making the structure heterogeneous. Such a structure generates local defects on the electrodes and the inter-actuator couplings. These local defects are translated by a line and grid effect that affects the reflected beam.
Another type of device is that in which two different torques are applied on either side of the mirror so as to cause an elliptical deformation of the mirror. The torques applied are generated by lever arms or strip springs. Such devices are not adapted, however, to thin mirrors of large dimensions. Indeed, it becomes very difficult to control any errors in the slope of the mirror in zones at a distance from the torque application points. Furthermore, the geometries made with such devices are necessarily simple geometry of the elliptical type. Nor do such devices allow the mirror's defects to be corrected, requiring the use of a mirror of excellent optical quality.